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By Robin Mitchell for Mouser Electronics

Published July 27, 2022

The term “volatile” is often used to describe something as sensitive, dangerous, or explosive, but its scientific meaning is somewhat divergent. In scientific terms, “volatile” refers to a substance that has a high vapor pressure such that a substance will readily turn from a solid or liquid into a gas. The degree to which a compound is said to be volatile relates to an environment at room temperature and standard sea level pressure. As such, volatile organic compounds (VOCs) are any organic compound that has a high vapor pressure.

Although the term “volatile” doesn’t necessarily mean explosive or combustible, most VOCs are flammable, and many can be explosive at low concentrations. As such, the sensitivity of VOCs presents engineers with significant challenges in environments exposed to an ignition source, where VOCs can be ignited by electrical contacts, switches, and even static electricity caused by clothing (e.g., gas station fires that result from jumping out of vehicles).

VOCs are widely found in industrial processes and in nature. Most of these VOCs are located in the oil industry at all stages, from drilling to refining. Examples of VOCs found in these environments include hydrogen sulfide and natural gas. Ethanol (alcohol) is another example of a VOC commonly found in nature through fermentation; however, risks from ethanol are generally introduced during distillation and not through natural fermentation.

VOCs can also be sourced from synthetic means, and such VOCs are commonly used as refrigerant thanks to their high vapor pressure. The compression, cooling, and subsequent vaporization of a VOC can be used to achieve low temperatures, which makes VOCs ideal for heat pumps.

A wide variety of VOC sensors are available to measure these VOCs, and each technology has distinct advantages and disadvantages. Implementing any VOC sensor technology must be done with significant consideration of the environment it will be used in as well as the nature of the manufacturing process.

Typical Applications for VOC Sensors

By far one of the most critical applications for VOC sensors is explosive gas monitoring. An environment that carries the risk of a VOC build-up will always require a gas detection system to sound alarms to anyone nearby. One typical example of such an environment is the oil industry, where hydrogen sulfide release can be deadly to those operating a rig (either through an explosion or poisoning).

VOC sensors are also helpful for gas leak detection. When attached to a portable wand or personal wearable, a VOC sensor can help engineers identify the potential source of a leak, whether it is a gas distribution installation, pipelines, boilers, or storage facilities.

The presence of VOCs in the air also affects air quality, especially inside buildings. As such, VOC sensors can be placed in air quality systems where the build-up of key VOCs can indicate poor air quality. Such a system can then be tied to a building's air conditioning system and pump fresh air from outside.

Finally, VOC sensors are critical in exhaust gas monitoring. A vehicle that combusts its fuel correctly will only produce carbon dioxide and water, but an engine that does not perform optimally will produce VOCs (among other things). As such, a VOC sensor can be used in test facilities to check the performance and efficiency of an engine.

Challenges in Using Sensor Technology to Measure VOCs

As previously stated, VOCs can carry a severe risk for explosion or fire; thus, any sensor used to measure a VOC must do so without igniting the VOC. Sensors that expose electrical components to VOCs can generate a spark under fault conditions, and such a spark could ignite the VOC. Therefore, direct sensing methods that expose conductors must either incorporate fire arrestors (i.e., systems that prevent an ignited mixture from causing a cascade effect) or ensure that sparks cannot form between conductors.

Additionally, VOCs' potential to be dangerous at low concentrations makes detection at these levels difficult. Trying to detect a compound at single parts-per-million levels presents a multitude of challenges for a sensor. VOCs are also highly reactive, which means that trying to detect a specific VOC is difficult if the sensor in question uses chemical binding (i.e., a sensor would only recognize the presence of a VOC not which VOC is present).

Types of VOC Sensors

Metal Oxide Semiconductor Sensors

One of the most common gas sensors on the market is the metal oxide semiconductor (MOS) gas sensor, which uses a direct sensing method whereby gases under detection make physical contact with the sensing material. To detect VOCs, MOS sensors use a small heating element that oxidizes the VOC. This oxidized compound then reacts with a metal oxide layer (usually tin oxide), which changes the layer's resistance.

While these sensors are often the cheapest and easiest to implement, they come with numerous challenges. First, MOS sensors use a small heater, which means they take time to heat up and become operational, and they cannot be switched on and off quickly. Second, these sensors can require up to 48 hours of settling time before they can be calibrated, creating challenges when working with a manufactured product.

Third, because MOS sensors react with organic and inorganic compounds, they offer little to no discrimination (i.e., they will detect the presence of all volatile compounds). This also means that they generally have poor accuracy and low sensitivity.

Fourth, using an onboard heater to oxidize VOCs presents an ignition risk. While many MOS sensors will include cages to prevent ignition, damaged MOS sensors could be extremely dangerous for environments that frequently expect VOC leaks.

PID Sensors

Photoionization detection sensors (PIDs) use a high-frequency light to break up VOC molecules, and the resulting broken molecules create an electric current that can be measured. PID sensors offer a high degree of accuracy, are sensitive to concentrations as low as 0.5ppb, and react to changes in concentrations in seconds.

Selectivity on PIDs can be partially achieved by using a specific frequency of light, which will provide a known amount of energy to each molecule (E=hf). Specific VOCs will have certain activation energies: A PID sensor will ignore VOCs under a particular energy but will react for those above this limit.

However, PIDs have difficulty with humidity, and their applications are limited owing to an inability to detect small VOC molecules such as methane. Furthermore, PID sensors generally work with functional groups, not hydrocarbon chains.

Electrochemical Sensors

Electrochemical sensors are similar to MOS sensors in that they oxidize a VOC to produce an electric current. While MOS sensors use a heating element to physically “combust” the gas, an electrochemical sensor uses a membrane that allows a VOC to diffuse (along with oxygen) and chemically combine at an activation site. This diffusion layer removes the explosive risk while allowing the sensor to operate at resolutions down to 10ppb.

Electrochemical sensors are cost-effective and have response times of around 30 seconds. Additionally, these sensors use a base voltage that allows for selectivity, which can be ideal for trying to identify a specific VOC. However, their construction means they have a short lifespan of less than two years (typical) and must be replaced frequently.

FID Sensors

Flame ionization detection sensors (FIDs) use a hydrogen flame that is placed in between two electrodes. Under nominal conditions, the hydrogen flame produces no ions as the hydrogen is fully combusted into water vapor. However, any VOC that burns under the flame will produce ions, which can be detected via electrodes, and the size of the resulting current represents the concentration of the VOC.

FIDs are advantageous in that they are low-cost, require low maintenance, and are extremely rugged to ensure proper operation. Furthermore, FIDs are extremely linear in that the current produced is proportional to the VOC concentration. However, FIDs present some disadvantages in that they are destructive sensors: The VOC being measured is destroyed and thus cannot be used where a VOC must be left unaltered. They also introduce an explosion risk as they rely on a hydrogen source that could leak if not installed correctly.

Photoacoustic Sensors

Photoacoustic sensors rely on the photoacoustic principle whereby absorbed light results in the production of sound waves. Essentially, a photoacoustic sensor uses an infrared (IR) emitter to heat a gas quickly, and the resulting soundwaves produced by the absorption of the IR light are detected with a small integrated microphone. While some photoacoustic sensors exist, they are somewhat of a rarity, and those that do exist are typically used for the detection of carbon dioxide. However, research is under way to produce photoacoustic sensors for use in VOC applications.

Photoacoustic sensors are advantageous in that they can distinguish between different gases that produce different sound waves. Additionally, the use of a selective light source presents a theoretical possibility for greater selectivity as some gases will absorb specific wavelengths of light better than others.

Design Integration Considerations

When incorporating a VOC sensor into an application, a multitude of design considerations need to be considered. Unlike most electronic components, VOC sensors can be extraordinarily sensitive to chemicals, temperature swings, and humidity in the environment. Additionally, many will have a plastic tab or cover to prevent damage to the sensor during manufacturing.

When incorporating a VOC sensor into a device, the temperature and humidity of the environment must be considered. Some VOC sensors will not operate correctly when exposed to temperature and humidity extremes, which can be disastrous in safety applications.

The gases that can be expected in the environment must also be considered. Some compounds (e.g., ammonia and nitrogen oxides) can trigger false alarms in VOC sensors. Furthermore, some compounds can even poison a sensor, meaning that its operation will no longer be reliable if exposed.

As a VOC sensor requires exposure to the monitored environment, vents in the sensor enclosure must be incorporated. The sensor and vent should also be positioned in such a way that gas can flow through easily (i.e., convection); otherwise, air trapped in the device could result in false measurements.

Sensors that produce an analog output must be mounted away from noisy circuitry (e.g., switch-mode power supplies and microcontrollers). Additionally, analog lines running from the sensor to a detector circuit must be careful not to allow high-speed signals to cross over the sensor lines (see electromagnetic compatibility and electromagnetic interference standard practices).

Finally, some VOC sensors will have holes to allow gas to diffuse in and out of the sensor, and these holes must be unobstructed. As such, engineers should pay close attention to protecting these holes from blockage by dust or dirt.

Choosing the Right VOC Sensor

Due to the potential (albeit unlikely) explosive risk of MOS sensors, they are best used in noncritical applications. For example, air quality systems used to monitor homes can take advantage of the simplicity offered by MOS sensors and their low cost. Furthermore, MOS sensors are small enough to be easily mounted inside low-profile enclosures.

PID sensors are ideal for industrial applications for which reliability and safety are required. Owing to their ability to detect minimal concentrations of VOCs, PID sensors are ideal for early warning alarms. Their energy selectiveness allows them to ignore VOCs under specific energies; however, they will still react to VOCs equal to or greater than their energy setting.

Electrochemical sensors are highly ideal for use in explosive environments thanks to their use of a diffusion layer. Their ability to detect low concentrations also makes them suitable for detecting leaks in crucial infrastructure such as pipelines and rigs. Still, electrochemical sensors must be designed to be easily replaceable due to their short lifespan.

Conclusion

VOCs present engineers with all kinds of challenges: They can be explosive, are difficult to detect in small concentrations, and come in many forms.

A wide variety of VOC sensors exists, and each technology has its advantages and disadvantages. MOS sensors are ideal for low-cost applications but struggle with humidity and can be an explosive risk, PID sensors do not provide great selectivity but can respond to very sudden changes in VOC levels, and electrochemical sensors provide excellent selectivity but must be replaced frequently. Implementing any VOC sensor technology must be done with significant consideration of the environment it will be used in as well as the nature of the manufacturing process.